EFFECT OF FIRST KREMER PRIZE
The announcement of the first Kremer Prize in 1960 spurred many projects to be started. The success of these projects varied widely. No review of human powered flight which covers this period is complete without a description of at least one of the large number of eager but inexperienced pioneers.
ALAN STEWART was building human-powered-ornithopters before 1959, and was still making attempts in 1979. In fact these were all unsuccessful Stewart-powered-ornithopters, although one glided once. He was perhaps the most persistent and notorious of the many young men of this period who had more optimism than aeronautical knowledge. The passage that follows shows the technical level of many of the enthusiasts of this period, and illustrates the organisational problems faced by anyone operating in the real world such as Stewart's village, Greenhill.It illustrates one of the necessary attributes. a strength of audacious pioneering despite the mockery of the neighbours. Perhaps the main lesson to be learned here is that on any project which is seen as bizarre, as HPF still is, it is necessary to develop a technique for coping with the curious.
To quote Alan Stewart :-
It wasn't long before someone leaned over the garden wall.
"What's that you're making then ? "
I tried to avoid giving a straightforward answer in the hope he would go away, but others appeared asking the same question.
"An aeroplane", I admitted at last, hoping that would get rid of them.
"Get away ! You are having us on. You never are ! Come on, tell us what you are making"
"An aeroplane", I repeated, preparing to meet their criticism with thick skin. At first they were puzzled.
People didn't make aeroplanes in their backyards.
"It's a practical joke !", grinned one at last, believing he had the answer. The others looked questioningly at me. I nodded. If they cared to think it was a joke, I wasn't stopping them. But as time went on, the people of Greenhill got to hear of the Kremer Prize and began to take me seriously. £5,000 was no joke. ( Stewart 1980 )
A different answer could have been something along the lines of Dr Paul MacCready's reply 18 years later to similar questioning at a time when he was as yet unsuccessful in muscle powered flight. (See Gossamer Condor.)
However, it wasn't Alan Stewart who lost out during a later series of questions, this time from the press.
Stewart again :-
The film sequence was carefully arranged....I was told to pedal as though really going somewhere. At a given signal I was to stop pedalling. This was a cue for the interviewer to duck under the wing and have a little chat with me. I pedalled until the wings began to fan up and down, the cameraman signalled me to stop and the interviewer came forwards, ducking smilingly under the wing to ask his first prepared question. "Tell me, Mr Stewart, don't you think this is all rather dangerous ?" But although I had stopped pedalling, the wings hadn't quite finished. They came steadily down to conclude the final beat. The main spar hit the interviewer on the head with such force that he fell to his knees. "It is a bit", I had to confess." (Stewart 1980)
PRESS ACCOUNTS cc1960
It was the less well-considered designs which were most written about and pictured in the daily press of this period, and the reporters found no shortage of material. Their vocabulary and accuracy however was more limited. Every overweight and under-wingspanned machine was pedalled "furiously", and every run was "an attempt on the Kremer Prize". In fact there was no official entry during the 1960s.
ROUGH ESTIMATE OF POWER REQUIRED
Sir Isaac Newton, Alan Stewart and those journalists in common did not understand the principle of the wing. Newton thought a wing worked by pushing air downwards. Since a propeller works by pushing air backwards, this would seem a reasonable assumption. A propeller creates a jet of backward moving air, but a wing creates a vortex around itself. A vortex is swirling fluid, as seen when a bath empties. At each wing-tip the vortex persists, and the wing leaves in its wake a pair of vortices. Thus the total vortex system is horseshoe shaped. At certain altitudes these trailing vortices give rise to observable vapour trails. These vortices are necessary for lift and will be present no matter how "perfectly" the wing is made. The creation of these vortices absorbs energy, and for efficient flight will be as small as possible. The beneficial effect of the vortex, the lift, acts all along the span so for the same vortex strength, the long wing produces more lift for the same energy. Hence, to generate the same lift a longer wing will need to generate a smaller vortex than a short wing, and will require less power from the pilot or engine. That is why success comes to HPAs with long wings. This energy loss manifests itself on the plane as a drag-force known as "induced drag". Added to the energy absorbed by this vortex generation will be the power which would be necessary to push the plane forward even if no lift were being generated. This is reduced by making it a smooth or "streamlined" shape, as for any vehicle. In practice it usually turns out that these two components of required power are of the same order of magnitude. Some optimisation procedures aim to make them equal The magnitude of this required power can be estimated at various degrees of sophistication as the project proceeds, but a rough estimate good enough to check the viability of a proposed design can be obtained as follows, from knowledge of five values :-
- Weight W total weight of pilot plus plane
- Span b distance wingtip-to-wingtip
- Area S planform area of wing
- Whether or not there is a fairing around the pilot
- Whether or not the wing is accurately made to a laminar-flow aerofoil-section,this involves extra weight of panelling and you must allow for this extra weight.
( Our 2011 formula supersedes the earlier version. )
Here are some computer lines which assume that the values are in ft lb sec units:-
kO=0.000020:IF Laminar THEN kO=0.000014
kF=0.0060:IF Faired THEN kF=0.0012
V=SQR(kV*W/S) :REM as on previous version
v3=V*V*V:t=b*b*V:IF t=0 THEN STOP
V is the speed at which you might expect to fly
P is the total pedalling power required from the pilot,
but remember that they are also having to steer the craft.
It is based on power to overcome Induced drag being K1 x W^2 / ( b^2 x V)
power to overcome Wing Profile Drag being K2 x S x V^3
power to overcome Fuselage Drag being K3 x V^3
The constants have been chosen such that the formula gives the right answer for machines that have actually flown.
Omitted here, is any reference to interference drag, propeller (in)efficiency or other factors.
This formula won't guarantee that you will fly, it will show clearly if you won't and it will show that you will if you get other things right too.
You can calculate using this formula or use Malcolm Whapshott's applet which uses it and allows you to to work in any units.
Click here for the calculator (opens in new window)
This estimate is good enough to show that most of the "planes" shown in the press of the 1960s on the ground were almost certain to stay there.
Stewart-1976 Span 30 feet, Weight 250 lbs, Area 180 sq ft
No fairing, Not laminar flow
V2 = ( 700 x 250 ) / ( 180 ) = 972
V = 31 ft/sec = 21 mph
P=2502 x 314/(302 x 31) + 313 x 180 x 0.000020 + 313 x 0.0060
P = 990 ft lb / sec ( nearly 2 HP or 1500 watts )
( the old version of this estimator gave 1076 )
The power that a person can produce depends on how long they have to produce it for, but however brief the flight, some time will be spent under exertion during the take-off run, say a total of half a minute. A person of average fitness can produce no more than 385 ft lb/sec for this time, and 990 is clearly more than 385.
Making the same calculation for Mufli and Daedalus, and comparing with more sophisticated estimates : -